1. Field of the Invention
The present invention relates generally to shutoff valves, and, more particularly, it relates to a shut-off valve which automatically blocks flow from a gas cylinder when the pressure in the cylinder falls below a predetermined level.
2. Brief Description of the Relevant Art
When the pressure in a gas cylinder is depleted, the cylinder must be refilled. It is desirable to leave a small residual pressure in the cylinder to prevent its contamination before it is refilled by the gas supplier. Gas cylinders are typically provided with a manual shut-off valve at their outlet, so that when the pressure in the gas cylinder falls below the minimum acceptable level, the user can close the hand valve to prevent a loss of the residual pressure.
Often, however, the manual shut-off valve on such cylinders is left open by the user and the residual gas pressure is lost. The cylinder must then be cleaned and purged prior to refilling in order to meet the required purity specifications for the particular gas. The cleaning of the cylinder is an extra step which increases the cost of the refilling.
It is thus desirable to provide an automatic shut-off means which will seal the outlet of the cylinder whenever the pressure therein falls below a predetermined level. Heretofore, this has been accomplished by providing a check valve which is typically located in the outlet port of the manual shut-off valve. Such check valves are disclosed in U.S. Pat. No. 3,384,133 to Gordon and U.S. Pat. No. 4,2lO,168 to Yonezawa.
However, the use of a check valve separate from the mechanism of the manual shut-off valve is undesirable for a number of reasons. First, it increases the complexity and cost of the valve. Second, it can be tampered with or removed by the user in an attempt to bypass the check valve. Further, an additional port is sometimes formed in the valve body where the user might attempt to remove the check mechanism while it is under pressure which can result in serious injuries. Additionally, in the gas cylinder industry, shut-off valves for gas cylinders must be made small enough to fit beneath protective covers of a standard size. The provision of a second valve mechanism within the shut-off valve body makes it more difficult to achieve a small enough valve size so that it fits inside a standard cover.
Nevertheless, the inclusion of a separate check valve is necessary because of the broad pressure range over which the cylinder operates. The manual valve must be capable of sealing against cylinder pressures which can be in excess of 3,000 PSIG, while the automatic shut-off of the cylinder should occur when the internal pressure is in the range from approximately 10 to 50 PSIG.
The combination of a valve plug and seat capable of forming a seal against very high pressures which is also suitable for automatic actuation by a spring set to seal against low pressures is disclosed in U.S. Pat. No. 4,402,340 to Lockwood. This patent discloses that a valve plug having a hard seal required for high pressure operation can be combined with a soft seal capable of establishing a low pressure seal in a manner which prevents the deformation and possible failure of the latter when subjected to high pressure. Thus, the Lockwood patent discloses a compact, automatically acting gas cylinder shutoff/check valve which will prevent low pressure leakage on cylinders even if they are stored for long periods of time prior to refilling.
The valves disclosed in the Lockwood and Yonezawa patents have been extensively tested. They both can be manually closed and they will hold high pressure. They also are designed so that they will automatically close and hold and maintain a residual pressure in the cylinder for an extended period of time.
They further require a mechanical system to hold the check valve in its open position prior to pressurizing the refilling manifold. Such a mechanical system differs from the standard Compressed Gas Association (CGA) connectors used in the industry. Thus, cylinders equipped with a valve disclosed in the Lockwood or Yonezawa patents require special handling while the special mechanical check valve opening adapters are installed.
Testing also indicates that if the check valves disclosed in the patents are pressurized at the valve outlet, thereby creating a large pressure differential with the valve inlet prior to the opening of the check valve (as might occur during cylinder refilling), the low pressure, soft seal can become dislodged, resulting in a failure of the check valve. The valves disclosed in these patents are therefore limited to a unidirectional high pressure differential from the valve inlet to the valve outlet.
Thus, it would be desirable to have a gas cylinder shut-off valve which includes a check valve and can be used with standard CGA connectors. It is, however, mandatory to provide such a valve with hard and soft sealing surfaces which can be operated irrespective of the direction of the high pressure differential without damaging either the hard or soft seal. Such a valve is not found in the prior art.
Valves having a single valve plug and seat and dual actuating means are found in other applications, however. U.S. Pat. No. 3,211,175 to Replogle discloses a combination regulating and check valve. An automatic actuator can be adjusted to provide the desired outlet pressure from the gas cylinder. An integral check valve is provided to allow the gas cylinder to be filled without having to remove the gas regulator from the cylinder. The disclosed valve is provided in addition to a manual shut-off valve. Consequently, it need not be capable to shut off over a wide range of pressure.
Further, a combination metering, check and shut-off valve is disclosed in U.S. Pat. No. 3,420,493 to Kraft. The valve described therein is a conventional needle valve having a spring loaded valve plug. A threshold flow of fluid through the valve holds the plug fully open in relation to an adjustable valve stem. Flow below this threshold level allows the valve plug to spring closed regardless of the position of the valve stem. This valve is also incapable of sealing over a wide range of pressure.
Thus, while it is known to combine a hard and soft sealing material into a single valve plug to provide a manually actuated high pressure seal and an automatic actuated low pressure seal, they have not been designed to allow a high pressure differential across the plug in both directions of flow.